The present invention relates to coating solutions comprising surface-modified nanoparticles.
A wide variety of products are produced by coating a solution onto a substrate. These solutions comprise at least one material that is a liquid at the coating conditions and, optionally, one or more materials that are solid at the coating conditions. Common liquid materials include, e.g., solvents (e.g., water, organic solvents and inorganic solvents) and syrups (e.g., monomers, oligomers, and polymers). Common solid materials include, e.g., resins (e.g., polymers), and fillers (e.g., particles and fibers). In some applications, a subsequent drying step is used to evaporate at least a portion of one or more of the liquid materials, allowing the solid materials to form a film on the coated surface of the substrate. Additionally, or alternatively, one or more of the liquid materials can be solidified by, e.g., curing and/or crosslinking to aid in the formation of a film.
Generally, it is desirable to produce a film that is uniform over the substrate, e.g., a film that is flat and level. In order to minimize film variations (e.g., film thickness), it is desirable to minimize defects. Defects may be generated during both the coating and drying operations. A wide variety of factors affect the formation of defects including: application method (e.g., roll coating, knife coating, and spray coating); drying method (e.g., forced-air and radiant energy); process conditions (e.g., line speed, temperature gradients, and coating thickness); coating composition; and coating properties (e.g., density, surface tension, viscosity, and solids content).
Efforts to reduce defects have included adding a surfactant to the coating solution, adjusting the solids content of the coating solution, and using less extreme drying conditions. Each of these approaches has drawbacks. The addition of a surfactant may compromise overcoating potential or interlayer adhesion, and introduce a species that may migrate through the dried coating layer. Alterations to the solids content may limit the window of operation for the coating process (i.e., the optimum solids content to minimize defects generated during drying may be very different than the optimum solids content for the desired coating operation). Less extreme drying conditions generally require reductions in line speed, which may impact the coating operation and increase production costs.